The present invention relates to a brake control system for use in a vehicle, more particularly to such a brake control system for a vehicle, which optimally controls the braking pressure on the braking operation of the vehicle on the basis of a dynamic model of a system relating to the brake of the vehicle.
Anti-skid control systems are known as systems for controlling the speed of each of wheels of a motor vehicle to maximize the frictional force between a tire and a road surface by causing the slip ratio SL [(vehicle speed--wheel rotational speed)/vehicle speed]to be controlled to assume 15 to 20%, with a view to preventing the vehicle from resulting in suffering decrease in safety due to locking of the wheels on vehicle braking operation, i.e., disabling of control of the vehicle, and from hip-swinging (skid) due to the locking of the wheels. One of example of such a system is disclosed in Japanese Provisional Publication No. 58-122248.
In anti-skid control systems of this type, to maximize the frictional force between the tire and road surface and allow the vehicle to quickly stop without causing the locking of the wheels, the rotational speed of each of the wheels is compared with a reference speed V derived on the basis of a vehicle speed Vs, i.e., given by V=Vs (1-SL) where SL represents slip ratio (0.15 to 0.2), and the braking hydraulic pressure is controlled so that the rotational speed of the wheel becomes equal to the reference speed. That is, with reference to FIG. 2, when the rotational speed V1 of the wheel is below the reference speed V, control is performed to decrease the braking pressure applied to the wheel, and when it exceeds the reference speed V thereafter, control is effected to increase the same. This braking pressure control is repeatedly performed, whereby the rotational speed of the wheel becomes closer to the reference speed V.
The brake of the wheel is normally achieved by applying braking hydraulic pressure generated under control to wheel braking cylinders so that the force corresponding to the braking hydraulic pressure is provided to braking members such as brake shoe and disc. Therefore, in the conventional brake control systems such as anti-skid control system, the control of the braking force has been performed by providing a hydraulic pressure adjusting device for the increasing, decreasing and maintaining of the hydraulic pressure in the hydraulic pressure system between a master cylinder operatively associated with a brake pedal of the vehicle for generating the hydraulic pressure and the wheel braking cylinders for directly controlling the braking members.
However, such conventional anti-skid control systems are arranged to individually control the rotational speeds of the respective wheels by controlling the braking hydraulic pressures to the respective wheels on the basis of so-called classic feedback control theory and therefore there arises problems that the lateral slippage of the vehicle occurs due to the unbalance of control caused by, for example, discrepancy in terms of abrasion between the tires of the wheels and difficulty is encountered to quickly stop the vehicle. Furthermore, with respect to each of the wheels, the conventional anti-skid control systems only makes possible to bring the rotational speed of the wheel approximately close to the reference speed due to the difficulty to be controlled so that the rotational speed is coincident with the reference speed and thus makes impossible to minimize the braking distance
In addition, the conventional brake control systems for vehicles have encountered the following probems, for which improvements should be required seriously.
(1) Control is performed to decrease the braking hydraulic pressure in response to the determination on the occurrence of the locking of the wheel, while control is effected to increase the braking hydraulic pressure in response to the requirement of the braking force due to the releasing of the locking. However, a dead time (response time lag) necessarily exists with such control performed through the hydraulic pressure system. Since it is impossible to detect the dead time, in the conventional vehicle brake control systems, the braking hydraulic pressure has been feedback controlled on the basis of the slip ratio of the wheel or the rotational angle speed thereof (controlled object), resulting in the occurrence of excessive control. That is, this causes the worsening of the drive feeling with overshoot and undershoot of the braking hydraulic pressure repeated during the anti-skid control for the vehicle.
(2) When the control of the braking hydraulic pressure is relaxed to eliminate the problem in terms of the excessive control with a view to ensuring fine drivability and drive feeling, there arises a problem in connection with the braking characteristics. This means that the braking distance would be made longer because difficulty is encountered to optimally control the slip ratio.
These problems (1) and (2) appear in the case that the pressure-decreasing of the braking hydraulic pressure is performed by varying the volume relating to a pressure-decreasing piston provided in the hydraulic pressure system, and when the control of the braking hydraulic pressure is effected by the valve opening/closing operations of an actuator such as solenoid-operated valve provided additionally, there would arise a problem associated with, particularly, dead time on control.